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Hello, the causes for that in order of most likely first are:
1. Water vapor condensed the pressure switch connecting tubes.
(remove tubes and blow out any condensate)
2. Cracked tube ends, so that they leak the vacuuum. (trim cracked ends)
3. Tiny orifice inside nipple where tubes connect is clogged (clean with
paper clip wire)
4. Bad diaphragm in pressure switch (older switches mostly, replace the
5. Blocked discharge air vent, or inducer blower wheel clogged with soot.
(remove and inspect)
6. Condensate trap open.
Tell me a lot about your technical skill sets so that I can coach you most effectively or determine the level of training needed.
We can go from there